Precision metal bellows are widely used in a number of applications where movement is required, but where sealing is also desired. For example, where it is desirable to have a vacuum on the exterior or interior of the bellows, the bellows provides environmental sealing. In other words, a bellows may be used as a boot for protective sealing. A bellows may also be used as a mechanical actuator by controlling pressure within the bellows, for example.
A bellows may be formed by joining together a series of bellows diaphragms in a predetermined pattern. A typical diaphragm is a generally circular disk with concentric folds formed therein. The diaphragms are joined together so that adjacent inside edges are connected together and adjacent outside edges are connected, etc.
The conventional approach to joining the diaphragms has been to tungsten inert gas (TIG) weld both the inside and outside joints. Unfortunately, TIG welding is relatively slow and may produce inconsistent quality welds.
In general, when TIG welding a pair of diaphragms are positioned in side-by-side relation and the interior weld is first formed using an inside diameter welding machine. Once a sufficient number of welded pairs or convolutions are made, these are assembled and positioned on a spindle or arbor. Copper "chill rings" or spacers are positioned between opposing outer portions. The chill rings help control the heating and heat dissipation of the diaphragms. The assembled convolutions and chill rings are rotated on the arbor, and each outer seam is TIG welded. The arc welding torch is indexed to each of the seams for welding. An operator watches the welding arc through a stereo microscope for alignment before and during welding. This operation may be very tedious for the operator. Moreover, the outcome of the bellows depends largely on the skills, experience, acuity, attentiveness, and the physical condition of the operator.
Once the outer welds are completed the structure is removed from the arbor, and the copper chill rings are then removed and discarded. Unfortunately, the copper chill rings must be made to relatively exacting tolerances and are therefore relatively expensive. Moreover, once used the copper rings may be discarded, and new rings may be supplied and used for making the next bellows. Accordingly, the cost of manufacturing is increased because of the cost of the copper chill rings.
Precision metal bellows manufacturing has been relying on gas tungsten arc (GTAW) or TIG welding for metal bellows for the past 20 years. Many improvements have been made to the process including power control, pulsing techniques and torch configuration. Unfortunately, the conventional arc welding process has inherent limitations in terms of productivity and quality. The welding speed is relatively slow (10-20 inches per minute) and the weld quality degrades as the welding tip wears out. Constant adjustments and finesse are needed to maintain the weld quality. In addition, the weld quality depends largely on the skill and the acuity of the operator. The process is labor intensive and is not well suited for automation. The costs of disposable and consumable items are also high.
U.S. Pat. No. 3,918,622 to Larsen discloses tungsten inert gas (TIG) welding of the outer joint of a plurality of stacked diaphragms and with optical tracking of a weld immediately following the formation thereof to maintain precise torch alignment with the weld joint. The diaphragms to be welded are stacked on a rotatable arbor. A radiation source such as a lamp, is focused by a lens and directed by a mirror to the seam. Reflected light from the seam is sensed by two side-by-side phototransducers so that a difference in the two output signals indicates the lateral displacement of the seam. Unfortunately, the split photosensor system described may not accurately indicate the position of the relatively small seams of many commercially desirable bellows.
Also relating to joining diaphragms to form a metal bellows, U.S. Pat. No. 3,626,582 to Melill discloses diffusion bonding for the stacked diaphragms. The fabrication requires pressuring fixture tooling including an external steel cylindrical retainer ring and an internal steel cylindrical retainer plug, together with a force transmitting annular ram and multiple thin steel pressure support rings using to transmit the bonding forces of the high pressure ram. The process, unfortunately, is relatively complicated and expensive at it requires elevated temperatures of 1600-1700 degrees F at 500 PSI pressure for 5 hours for typical thin sheet metal titanium diaphragms.
Laser and electron beam apparatus have also been disclosed for welding the joints of metal bellows. For example, U.S. Pat. No. 4,760,236 to Stoll discloses a laser welding apparatus wherein the outer diameter welds are made one at a time to form pairs of diaphragms. The pair of diaphragms are pressed between opposing tooling fixture portions to ensure alignment. To form the inner welds, a series of pairs are positioned within an evacuated chamber and the laser beam is directed through the center opening to the inner weld positions. For both the inner and outer welds, the laser beam is directed at an oblique angle. Unfortunately, an oblique angle may cause a nonsymmetrical and low quality weld. Moreover, precise positioning of the laser beam for the inside welds may be difficult and result in poor quality welds.
U.S. Pat. Nos. 5,478,983 and 5,410,123 both to Rancourt disclose an apparatus for forming a bellows bladder using a laser beam to form the inner and outer welds. An oblong shaped laser spot is used for welding. In addition, a comb structure is used to separate and position interior joints and exterior joints for laser welding. Precise relative positioning requires high accuracy of the comb and little or no variations in the convolutions. In short, precise positioning of the laser beam relative to the weld areas may be difficult to achieve.